Caramel can be made in two ways: by heating plain sugar or by heating a sugar syrup. The latter method is the more successful as syrup is less likely to burn and the color will be more even.
To caramelize sugar syrup: boil syrup rapidly until it starts to brown around the edge of the pan. Lower the heat and continue cooking, swirling the pan once or twice so the syrup colors evenly.
Tip the pan sideways during cooking to check the color of the caramel as it runs off the bottom of the pan. Here, light caramel is shown.
For dark caramel, continue cooking, watching the syrup closely. When smoke starts to rise, the caramel is done.
To stop caramel from cooking further in the heat of the pan, plunge the base of the pan in a bowl of cool water.